Criteria for Selecting Server Hardware
Before you think about processor speeds, do some planning to determine what you'll be doing with the server. Here are the two key issues: how many users access the server and what the users do with it. Neither tells the whole story by itself; you must consider both. When you have this information, your hardware options become clearer.
Number of users
The effect of an increasing number of connected clients on server performance isn't linear. You may not notice slower service as the number of connected clients increases until you get to a tipping point, when performance suddenly slows to a crawl.
The Mac mini can handle a maximum of around 20 to 50 simultaneous client computers doing lightweight tasks. The more hardware-intensive services you run, the lower that number is. With up to ten or so users, the lower-end Macs can handle multiple tasks at once. If you add more users later, you can always add more lower-end Mac servers for other tasks. Current iMac models have faster processors and architecture than the minis, can hold more RAM, and can handle more clients.
The top-of-the-line Mac Pro can potentially handle hundreds of clients, again depending on what services you run. With the Mac Pro or an older Xserve, adding more higher-end storage or large amounts of RAM can help enlarge the client load that the server can handle. Network capacity (number of Ethernet cards and their speed) is also important in serving large numbers of clients.